The initial step in the recovery of hydrocarbons and gases from underground formations is the placement of the cement slurry, usually including cement, water and other additives, in the annular space between the porous formation and the steel casing. The main purpose of the cement in this annular space is to support the casing and also restrict fluid movement between formations. This process is referred to as primary cementing. The most important requirements to insure a satisfactory cementing job are that a highly homogenous cement is present in the annulus and that strong bonds develop between the rock formation, cement and steel casing.
In order for this to happen, one must insure that the composition of the cement slurry pumped downhole does not change greatly during the whole cementing process. Therefore, contamination of the cement slurry by the drilling mud used in the previous operation or excessive loss of water to the formation must be carefully avoided. A well-formulated cement slurry will, therefore, exhibit low fluidloss to the formation, give either zero or very low free-water, have a viscosity low enough to be pumpable, set to a hardened mass within the desired time interval and provide adequate compressive strength to support the casing.
The free water of a slurry is simply the supernatent fluid formed on top of the slurry column which provides an indication of the amount of settling of the cement particles during slurry placement. Excessive free water on top of the cement column will result in an incompetent zone close to the top of the liner which will have to be remedied with an expensive squeeze job. The viscosity of the slurry describes the rheological behavior of the slurry, which is determined by measuring the plastic viscosity (pv) and the yield point (yp) of the slurry. The cement slurry should be fluid and pumpable until it is in place, then it should start to set as soon as possible after placement. Any delay in the development of compressive strength will increase the "waiting on cement" time (WOC) necessary before proceeding with the next operation. The thickening time (TT) is used to describe the point at which the gelation of the cement has proceeded to such an extent so as to affect the pumping rates.
Neat cement slurries have a fluidloss which varies from 700 to 2500 mL over a thirty minute period. This rate of loss will result in rapid dehydration and incorrect placement of the slurry and consequently will lead to failure of the whole cementing job. In order to attempt the control of fluid loss from the cement slurry to the surrounding rock formation, the permeability of the cement matrix must be reduced. This is achieved by addition of additives which provide excellent fluid loss control but at the same time do not adversely affect other properties of the slurry, such as free water, rheology, thickening times and compressive strength.
Commercial fluid loss additives based on cellulosic polymers such as hydroxyethylcellulose (HEC) provide fairly good fluid loss control but there some obvious drawbacks associated with cellulosic polymers. Typical loadings of these polymers to achieve good fluid loss control are in the range of 0.5 to 1% by weight of cement. Such high loadings results in a dramatic increase in the plastic viscosity and the yield point of the slurry, which translates to an increase in the energy needed to pump the slurry downhole. This means that every improvement in the fluidloss brought about by an increase in the level of the cellulosic polymer has to be paid for with high pumping pressures. Other drawbacks of cellulosic polymers include retardation in the thickening times of slurries and also the instability of these polymers at high temperatures, which limits their usefulness as a fluid loss additive to wells cooler than 200 degrees F.
Synthetic polymers based on acrylamide and polyvinylpyrrolidone have been considered by the industry as additives for fluid loss control in cement compositions, but they have not come into wide spread use because of certain inherent drawbacks. Both acrylamide and polyvinyl pyrrolidone are prone to hydrolysis in the alkaline environment of the cement composition and therefore cause excessive retardation in the development of cement compressive strength. Furthermore polyvinylpyrrolidone is expensive and has a strong flocculating behavior which makes it an unattractive candidate as an additive in cement compositions. The synthetic polymers used as fluid loss additives are typically high molecular weight polymers which are quite expensive and this limits the amount of polymeric additive the can be added to the cement composition. Even if the cost factor is overlooked, high loadings of synthetic polymers may give improved fluid loss properties but often lead to viscous slurries which require increased pumping energy.
The art is replete with copolymers which function as additives in cement compositions, but the inherent drawbacks associated in the form of undesirable side effects in the case of cellulosic and some synthetic polymers, coupled with the high cost of synthetic polymers have severely curtailed their widespread use in the industry.
As heretofore noted, slurry compositions for oil and gas well cementing typically require that the slurry be fluid enough to be pumpable. This means that the plastic viscosity of the slurry should be less than 100 centipoises (cps), and more preferably less than 50 cps. In addition, the yield point of the slurry should be less than 20 lbs/100 ft.sup.2, the fluid loss be less than 50 mL/30 minutes and the freewater be less than or equal to 3 mL over a two hour standing period. Cementing compositions meeting these requirements can be extremely difficult to design, and often times do not meet all of these requirements.
Certain cementing compositions which contain anti-gas channeling additives have been described in the art. For example, Parcevaux et al., U.S. Pat. No. 4,537,918, describes a cement composition which contains styrene-butadiene latex, as well as a stabilizer. The stabilizer is described as an anionic component of the cement composition which is selected from the group of lignosulfonates and their partly desulfonated derivative, sulfonic acid or sulfite modified melamine-formaldehyde resins, formaldehyde/sulfonate naphthalene resins, and condensation products of bi-nuclear sulfonated phenols and of formaldehyde. Each compound in this recited class of stabilizers is thus required to be anionic and to include a compound which contains sulphur in its molecule.
In Parcevaux et al., U.S. Pat. No. 4,721,160, there is also described a cement slurry composition which contains a styrene butadiene latex and a stabilizer. However, the patentees limit their additive for use with a resulting cement composition having a specific gravity within the range of 1.2 to 1.6, i.e. a lightweight cement slurry composition.
In addition, U.S. Pat. No. RE 28,722 reissued to Sanders, U.S. Pat. No. 3,043,790, describes a cement mortar composition which includes a styrene butadiene additive system having three necessary additives. These include a nonionic surfactant, an ionic surfactant and a polyorganosiloxane fluid surfactant. The preferred nonionic surfactants include condensation products of ethylene oxide with phenols, naphthols, and alkyl phenols, for example octyphenoxy- nonaoxyethyleneethanol. There appears to be no mention made of the use of this particular composition in oil and gas well cementing applications.
There presently exists a need in the art for an additive system which is relatively inexpensive, and provides superior fluid loss control via a synergistic combination of nonionic and anionic surfactants, without adversely affecting other critical properties of the cement slurry for oil and gas well cementing.